Although last minute reservations are always possible, it is best to reserve your car rental about 3 months in advance. There are several reasons for this:
1- By booking early, you are assured of having availability for the selected dates. Indeed, several types of vehicles are not available at the last minute such as minivans and SUVs.
2- The earlier you book, the more you save. Indeed, the closer you are to the departure date, the less availability there is. The law of supply and demand dictates that the less availability, the higher the price.
Peak season in Portugal
These 2 good reasons to book early are especially true if you are traveling during high season.
The high season in Portugal is the summer months from June to August.
If you want to enjoy milder weather and less tourist traffic, spring and fall are best. The best time to travel is from March to May and from September to November.